1. Field of the Invention
The present invention relates to a light transmitting apparatus constructed to suppress backward scattered light (stokes light) caused due to stimulated Brillouin scattering (SBS) which occurs on a light transmission path.
2. Description of the Related Art
It has hitherto been known that stimulated Brillouin scattering (SBS) light is generated when a semiconductor laser diode (LD) of a light transmitting apparatus transmits an optical signal having a small emission spectral width to a light transmission path. When the stimulated Brillouin scattering light is generated, incident light turns out to be stokes light and travels back to the light transmitting side due to a conversion of the light propagated in the reverse direction. Optical output electric power transmitted in the form of the stokes light propagated in the reverse direction is reduced, and a quality of data transmission thereof declines. As a result, a data error rate rises, and there arises inconveniences such as a disconnection of the line.
The stimulated Brillouin scattering light occurs when over a certain value of optical output electric power (when over a threshold value). The threshold value at which the stimulated Brillouin scattering light occurs becomes in this case higher with a larger emission spectral width of a light source such as a laser diode of the light transmitting apparatus. Accordingly, for implementing a stable data transmission while avoiding the occurrence of the stimulate Brillouin scattering light, it is effective to set the threshold value high by enlarging the emission spectral width of the light source (such as the LD etc) of the light transmitting apparatus.
The enlargement of the emission spectral width involves adding frequency modulation on the driving current of the laser diode. If a frequency modulation component is overlapped with the driving current of the laser diode, a waveform of a light transmission output to be transmitted is liable to deteriorate. Accordingly, in the case of superposing the frequency modulation component on the driving current of the semiconductor laser diode, it is required that the superposition thereof be set to the minimum.
For restraining down to the minimum the deterioration of the transmission characteristic which is caused by both of the frequency modulation and the stimulated Brillouin scattering light, it is required that a quantity with which to enlarge the emission spectral width of the light source such as the laser diode be adjusted (set) to a proper value corresponding to a light intensity of the stokes light while observing the stokes light produced due to the stimulated Brillouin scattering light.
What is known as this type of prior art light transmitting apparatus is disclosed in Japanese Patent Laid-Open Publication Nos. Hei 7-28107 and Hei 8-6078.
FIG. 1 is a block diagram showing a construction of an optical amplifier in the prior art.
This optical amplifier is constructed to detect noise light propagated in the direction opposite to that of an optical signal. The detection of the noise light involves observing amplified spontaneous emission (ASE) generated and outputted by a pumping light source. An optical circulator is disposed between two rare-earth doped fibers in order to detect the amplified spontaneous emission (ASE). A photo detector detects the amplified spontaneous emission from the optical circulator.
FIG. 2 is a block diagram illustrating another construction of the optical amplifier in the prior art.
An external modulation light transmitting apparatus in this prior art involves the use of an optical splitter and an optical combiner in order to detect an optical beat signal (optical interference) between transmission signal light and backward scattered light (stokes light) due to stimulated Brillouin scattering light. The detected optical beat signal is fed back to a driving circuit of a phase modulator, thereby effecting phase modulation on the signal light from the semiconductor laser diode. With this phase modulation effected, an emission spectral width of the semiconductor laser diode is controlled.
In the former prior art, the optical circulator disposed between the rare-earth doped fibers detects the amplified spontaneous emission. In this case, the stokes light generated due to the stimulated Brillouin scattering on the light transmission path is absorbed by the rare-earth doped fibers. Hence, there arises such a defect that the stokes light can not be detected through the optical circulator.
In the latter prior art, the optical splitter and the optical combiner are needed for detecting the optical beat signal between the stimulated Brillouin scattering light and the optical signal. Consequently, the number of the optical circuit parts increases, resulting in a defect of causing scale-ups of the apparatus and of signal processing.